It has been proposed in U.S. Pat. No. 5,866,282 Bourdelais et al., to utilize a composite support material with laminated biaxially oriented polyolefin sheets as a photographic imaging material. In U.S. Pat. No. 5,866,282, biaxially oriented polyolefin sheets are extrusion laminated to cellulose paper to create a support for silver halide imaging layers. The biaxially oriented sheets described in U.S. Pat. No. 5,866,282 have a microvoided layer in combination with coextruded layers that contain white pigments. The composite imaging support structure described in U.S. Pat. No. 5,866,282 has been found to be more durable, sharper and brighter than prior art photographic paper imaging supports that use cast melt extruded polyethylene layers coated on cellulose paper.
It has been proposed in U.S. Pat. No. 5,744,287 that digital printing artifacts could be reduced by improvements of the image support materials. Improvements such as sharpness and minimum density would reduce certain digital printing artifacts and improve image quality.
Of the artifacts associated with printing digital images onto silver halide media, formation of visually soft or "bleeding" edges, especially around text, probably elicits the greatest objections. In the current invention this artifact is designated "digital fringing", and it pertains to unwanted density formed in an area of a digital print as a result of a scanning exposure in a different area of the print, not necessarily in adjacent pixels. Digital fringing may be detected in pixels many lines away from area(s) of higher exposure, creating an underlying minimum density or Dmin that reduces sharpness and degrades color reproduction. It should not be confused with system flare arising from improper calibration, which produces a similar macroscopic defect.
Digital fringing may be observed even with exposures producing mid scale densities. The minimum exposure at which digital fringing becomes visually objectionable varies by digital printing device and emulsion photographic properties. Because fringing increases with exposure, the useful density range for typical commercial color photographic papers printed by scanning laser or LED (light emitting diode) exposures must be restricted to 2.2 or below, less than the full density range of the papers. Fine line images require even lower print densities due to the acute sensitivity of the eye to softening of high contrast edges.
Other image artifacts associated with optical scan printing on silver halide media that should not be confused with digital fringing are "contouring", "banding", and "rastering". "Contouring" refers to the formation of discrete density steps in highlight regions where the gradations should appear continuous. Bit limited system modulators (those that use .ltoreq.2.sup.10 bits, or 1024 DAC levels, designated 10 bit), may have too few levels to calibrate for density differences that are below the detection threshold of the human eye. A single bit change in exposure may, therefore, produce a density change large enough to see as a step, or contour. Lower contrast toe regions of the paper H&D curves can alleviate contouring in a 10 bit system, as taught by Kawai, Kokai JP 05/142712-A, but the low contrast also lowers the density threshold for digital fringing. System modulators using 2.sup.12 bits (designated 12 bit, having 4096 DAC levels) are not as susceptible to contouring artifacts. "Banding" is the appearance of lines, or bands, having a lower frequency than the individual raster lines, but which are parallel to the line scan direction. The bands arise from non-uniformity in the overlap exposure between scans (e.g., from mechanical vibrations) causing fluctuations in exposure in the overlap areas large enough to produce a visually detectable difference in density. "Rastering" is a high frequency artifact related to non-optimal spot size or shape which allows the eye to resolve the individual scan lines.
Those skilled in the art will recognize that the optical properties of the media (the scattering of light by the emulsion layers and predominately the paper base) contribute in part to digital fringing, which is a loss of acutance or sharpness. A general discussion of acutance as it pertains to structure of photographic media can be found in Mees & James, The Theory of the Photographic Process, 4th Edition, Chapter 21. The spot shape and spot size used in scanning laser exposures also contribute to loss of sharpness.
Typical chemical negative working color papers are restricted with regards to tone scale or contrast levels due to the fact that the paper must print acceptably with a color negative. Digital only color papers, however are not restricted as the digital printing device can electromodulate the curve shape to provide an acceptable tone scale in the image. It would be desirable to have a color paper that would provide not only excellent tone scale to optimize image quality in a negative working system, but also provide sufficient reciprocity control to survive the extremely short exposure times typical in sub microsecond digital exposure devices.